A pacemaker is a medical device, typically implanted within a patient, which provides electrical stimulation pulses to selected chambers of the heart, i.e., the atrium and/or the ventricle. Such stimulation pulses cause the muscle tissue of the heart (myocardial tissue) to depolarize and contract, thereby causing the heart to beat at a controlled rate. Most pacemakers can be programmed to operate in a demand mode of operation, i.e., to generate and deliver stimulation pulses to the heart only when the heart fails to beat on its own. To this end, the pacemaker senses cardiac activity, i.e., heart beats, and if the heart beats do not occur at a prescribed rate, then stimulation pulses are generated and delivered to an appropriate heart chamber, either the atrium or the ventricle, in order to force the heart to beat.
When operating in a demand mode of operation, the pacemaker defines a period of time, referred to generally as the “escape interval” (which may further be referred to as either an “atrial escape interval” or a “ventricular escape interval,” depending upon the mode of operation of the pacemaker) that is slightly longer than the period of time between normal heart beats. Upon sensing such a “natural” (non-stimulated or non-paced) heart beat within the allotted time period, the escape interval is reset, and a new escape interval is started. A stimulation (or pacing) pulse is generated at the conclusion of this new escape interval unless a natural heartbeat is again sensed during the escape interval. In this way, stimulation pulses are generated “on demand,” i.e., only when needed to maintain the heart rate at a rate that never drops below the rate set by the escape interval.
The heart rate is monitored by examining the electrical signals that are manifest concurrent with the depolarization or contraction of the myocardial tissue. The contraction of atrial muscle tissue is manifest by the generation of a P-wave. The contraction of ventricular muscle tissue is manifest by the generation of an R-wave (sometimes referred to as the “QRS complex”). The sequence of electrical signals that represent P-waves, followed by R-waves (or QRS complexes) can be sensed from inside of or directly on the heart by using sensing leads implanted inside or on the heart, e.g., pacemaker leads; or by using external electrodes attached to the skin of the patient.
Most modern implantable pacemakers are programmable. That is, the basic escape interval (atrial and/or ventricular) of the pacemaker, as well as the sensitivity (threshold level) of the sensing circuits used in the pacemaker to sense P-waves and/or R-waves, as well as numerous other operating parameters of the pacemaker, may be programmably set at the time of implantation or thereafter to best suit the needs of a particular patient. Hence, the pacemaker can be programmed to yield a desired performance.
The operation of a pacemaker as described above presupposes that a stimulation pulse generated by the pacemaker effectuates capture. As used herein, the term “capture” refers to the ability of a given stimulation pulse generated by a pacemaker to cause depolarization of the myocardium, i.e., to cause the heart muscle to contract, or to cause the heart to “beat.” A stimulation pulse that does not capture the heart is thus a stimulation pulse that may just as well have not been generated, for it has not caused the heart to beat. Such a non-capturing stimulation pulse not only represents wasted energy—energy drawn from the limited energy resources (battery) of the pacemaker—but worse still may provide the pacemaker logic circuits with false information. That is, the logic circuits of the pacemaker may presuppose that each stimulation pulse generated by the pacemaker captures the heart. If the stimulation pulse does not capture the heart, then the pacemaker logic circuits control the operation of the pacemaker based on false information, and may thus control the pacemaker in an inappropriate manner. There is thus a critical need for a pacemaker to properly determine whether a given stimulation pulse has effectuated capture. Failure of a pulse to effectuate capture is referred to as a “loss of capture” or LOC.
While there are many factors that influence whether a given stimulation pulse effectuates capture, a principal factor is the energy of the stimulation pulse. The energy of the stimulation pulse, in turn, is determined by the amplitude and width of the stimulation pulse generated by the pacemaker (the “output settings”) and the electrical resistance of the pacemaker system/tissue interface circuit. Advantageously, in a programmable pacemaker, both the amplitude and pulse width of the stimulation pulse are parameters that may be programmably controlled or set to a desired value. Typically, over the lifetime of a device, the Pulse Width is left at a fixed value (usually 4 or 5 milliseconds) and the Amplitude (Voltage) is varied to adjust for changing capture requirements.
An implantable pacemaker or ICD derives its operating energy, including the energy to generate a stimulation pulse, from a battery. The energy required to repeatedly generate a stimulation pulse dominates the total energy consumed by a pacemaker. Hence, to the degree that the energy associated with the stimulation pulse can be minimized, the life of the battery can be extended and/or the size and weight of the battery can be reduced. Unfortunately, however, if the energy associated with a stimulation pulse is reduced too far, the stimulation pulse is not able to consistently effectuate capture, and the pacemaker is thus rendered ineffective at performing its intended function. It is thus desirable for a pacemaker to adjust the amplitude of a stimulation pulse to an appropriate level that provides sufficient energy to effectuate capture, but does not expend any significant energy beyond that required to effectuate capture.
Initially, the most common technique used to adjust the stimulation amplitude to an appropriate level was manually, using the programmable features of the pacemaker. That is, at the time of implant, a cardiologist or other physician conducts some preliminary stimulation tests to determine the amplitude a given stimulation pulse must have to effectuate capture at a given tissue location. This stimulation pulse amplitude is hereafter referred to as the “capture threshold.” If the preliminary tests indicate that the capture threshold is high (compared to the average patient) then the lead will be repositioned until a “good” threshold is found. Once it has been determined that the thresholds are acceptable, the stimulation electrode is then left in place and the amplitude and/or width of the stimulation pulse is set to a level that is typically 2 to 3 times greater than the amplitude determined in the preliminary tests. The increase in amplitude beyond the measured pacing threshold is considered as a “safety margin.”
During the acute phase, e.g., over a period of days or weeks after implant, the capture threshold usually increases significantly over that measured at implant. Hence, having a safety margin factored into the stimulation pulse amplitude allows the stimulation pulses generated by the pacemaker to continue to effectuate capture despite this acute change in the capture threshold. Unfortunately, however, much of the energy associated with the safety margin represents wasted energy, and shortens the life of the battery. Near the end of the acute phase the threshold typically returns back down to a value near the level measured at implant. After the acute phase the lead enters the chronic phase. Typically, during the chronic phase the threshold remains relatively stable compared to the acute phase. If left unchecked, the safety margin determined necessary at implant is extremely wasteful during the chronic phase. Thus, once the lead enters the chronic phase, typically the pacing output level is adjusted lower, but never less than 2-3 times threshold.
Some single-chambered pacemakers (e.g. pacemakers that pace in the left ventricle only) and dual-chambered pacemakers (e.g. pacemakers that pace in both the left ventricle and left atrium) now include an automatic stimulation threshold search system which, following implant of the pacemaker, automatically determines the minimum capture threshold and sets the stimulation pulse amplitude accordingly. The stimulation threshold search system periodically re-determines the threshold and re-sets the stimulation pulse amplitude, if needed. Hence, if the threshold increases with time, the stimulation pulse amplitude is increased as needed to maintain capture. If the threshold decreases with time, the stimulation pulse amplitude is decreased as needed so that energy is not wasted. These automatic features represent an alternative to manual adjustment of threshold. Because the pacing threshold is frequently checked, a smaller safety margin can be used. In dual-chambered pacemakers, the stimulation threshold search system is typically applied only to ventricular pacing pulses.
In the St. Jude Medical Autocapture system, to periodically determine the capture threshold, the “stimulation threshold search” system applies a sequence of stimulation pulses to the heart tissue with differing pulse amplitudes and determines the lowest amplitude sufficient to effectuate capture. Autocapture is a registered trademark of St. Jude Medical, Inc. In the Autocapture system, an “automatic capture verification” system looks for an evoked response following the pulse to determine on a beat by beat basis if capture has been effectuated by a stimulation pulse. If an evoked response is detected, capture is thereby verified. If no evoked response is detected, a backup pulse is delivered and a new stimulation threshold search is performed to determine a new capture threshold. The pulse amplitude is then set to a level just above the threshold, a function termed “Automatic Output Regulation.” Herein, the term “Autocapture” is used to refer to systems that provide for beat by beat automatic capture verification with a backup safety pulse and automatic stimulation threshold search with the automatic output regulation.
More specifically, Autocapture provides for:                automatic Capture Verification, which monitors every paced beat for the presence of an evoked response;        automatic Loss of Capture Recovery, which in the absence of an evoked response triggers an automatic backup safety pulse to ensure capture, ensuring patient safety;        automatic stimulation threshold search, which, due to various triggers initiated by the device algorithms, measures pacing thresholds to determine the pulse amplitude requirement; and        Automatic Output Regulation, which sets the pulse amplitude to an “operating amplitude” just above the measured threshold, ensuring the lowest energy level that ensures capture and thus optimizing device longevity.        
An example of an Autocapture system is described in detail within U.S. Pat. No. 5,417,718 to Kleks et al., entitled “System for Maintaining Capture in an Implantable Pulse Generator”, which is incorporated by reference herein.
Autocapture has proven to be highly successful within single-chambered pacemakers and dual-chambered pacemakers. However, heretofore, Autocapture has not been applied to biventricular pacing devices, i.e. dual-chambered pacemakers that have the capability for delivering synchronized pacing pulses to the left and right ventricles. Biventricular pacemakers have shown the ability to increase the performance of patients with congestive heart failure (CHF) by synchronizing the contraction between the left and right side of the heart. Autocapture has not been employed within biventricular pacing devices, in part, due to the potential for an evoked response from one ventricle, for instance the left ventricle, to create a false positive evoked response signal on the opposite channel—the right ventricular (RV) channel. For example, within a biventricular system providing separate sensing and pacing leads in the left and right ventricles, the portion of the R-wave generated by depolarization of the left ventricle in response to the a Left ventricular (LV) pacing pulse will propagate into the right ventricle where it might be sensed by the RV sensing lead mounted therein. If it is sensed in the right ventricle after a separate pacing pulse is delivered to the right ventricle, the device may misinterpret the sensed signal as being representative of depolarization of the right ventricle triggered by the RV pacing pulse. Hence, even if the amplitude for the RV pacing pulse is below the threshold at which it will evoke a response in the right ventricle, the device will nevertheless assume that the pulse amplitude is sufficient and that the pulse had properly captured the right ventricle. This phenomenon is referred to herein as “cross channel evoked response sensing”.
As a result, it does not appear that Autocapture has ever been provided within commercially available biventricular pacing systems. Hence, patients with implantable biventricular pacing devices do not gain the benefits of the Autocapture technique. As noted above, without Autocapture, the amplitude of individual pacing pulses must typically be set to a sufficiently high level to substantially guarantee that each pulse will be captured, thereby consuming a greater amount of energy from the battery of the implantable device and hence diminishing device longevity and requiring more frequent replacement. Energy consumption within a biventricular device is particularly critical in view of the need to deliver pacing pulses to both left and right ventricles and in view of the generally higher pulse amplitudes required within the left ventricle. Also, there is always a risk that pacing pulses will not properly be captured, perhaps because the physiological threshold needed for evoking a response within the heart of the patient has increased with time as a result of disease progression, medication, lead dislodgment, or other factors. Hence, without automatic capture verification, there is a risk that biventricular pacing therapy needed by the patient will not properly be delivered to the patient with potentially serious consequences.